1. Field of the Invention:
The present invention relates to a magnetic bearing controller for compensating for displacement of a rotor axis in a non-contact manner utilizing magnetic force, and in particular to improved control of an amount of current to be supplied to windings.
2. Description of the Related Art:
FIG. 2 is a perspective view showing a general structure of a magnetic bearing. A rotation motor 3 causes the axis 1 to rotate at a high speed. A magnetic bearing 4 for a thrust direction holds the axis 1 for a constant position in the thrust direction. Magnetic bearings 2, 5 for a radial direction hold the axis 1 in a constant position in the radial direction.
FIG. 3 is a block diagram showing a structure of a conventional magnetic bearing controller for holding the axis in a constant position in the radial direction. In the drawing, the radial magnetic bearing is shown cross sectionally, viewed in the thrust direction. The rotor 6 is a cylinder formed by laminating ring-shaped silicon steel sheets in the thrust direction, and fixed to the axis 1 by shrinking insert process. The stator 7 is formed by laminating silicon steel sheets in the thrust direction, in which eight pole teeth are arranged equidistantly along the inner circumference thereof. As two adjacent teeth make one pair, there are four pairs in the configuration of this embodiment. Each pair is wound by a winding 11, 12, 13, 14, forming four electromagnets. Here, when current is supplied to any winding, an excited electromagnet generates a magnetic attraction force, which in turn causes the rotor 6 to move in the direction of the excited electromagnet.
Position detectors 8, 9 detect the position of the rotor 6 in the X- and Y-axis directions, respectively, in a vertical coordinate system, and output position signals SX, SY. Subtractors 15, 16 subtract the respective position signals SX and SY from target position signals GX, GY for the X and Y directions in the rotor 6, and output corresponding subtraction signals DX, DY. Subtraction signals DX, DY are input into respective force control circuits 17, 18, respectively, which in turn output force command values FX, FY in the X and Y directions, respectively. The force command values FX, FY to be given to the rotor 6 are output so that subtraction signals become zero. Current value converters 19, 20 carry out appropriate processing, such as a square root operation or a bias thrust addition, with respect to the force command values FX, FY such that the force command values FX, FY have linear correspondence with current to be supplied to the windings, to thereby convert the force command values FX, FY into current value signals CPX, CNX, and CPY, CNY, indicative of the value of a current to be supplied to the windings 11, 13 and 12, 14, respectively. Current control circuits 21, 22 perform pulse width modulation to drive a number of incorporating power transistors to respectively control currents IPX, INX and IPY, INY to be supplied to the windings 11, 13 and 12, 14 so as to match the currents with the current value signals CPX, CNY and CPY, CNY.
In operation, the magnetic bearing controller shown in FIG. 3 can control the rotor 6 such that the positions thereof in the two directions in the vertical coordinate system will follow the target position signals GX and GY, respectively. When the target position signals GX and GY are held constant, the magnetic bearing can function as a radial bearing for restraining the position of the rotor 6 in the radial direction.
However, a conventional radial magnetic bearing has a problem of increased cost as a magnetic bearing controller requires a number of expensive power transistors to control eight independent phases of currents to be supplied to the four electromagnets.
Further, because each of the eight pole teeth must be wound by a winding, manufacturing steps are resultantly complicated and costs are further increased.
Still further, because, except for magnetic bearings, there exists almost no device requiring control of eight-phase currents flowing in four windings, and a magnetic bearing is almost exclusively used in a specific spindle for a vacuum pump or a machining tool, and so on, for high speed rotation, scale merit of cost reduction can be hardly expected. Thus, very expensive current controllers are resulted. Therefore, a magnetic bearing system, despite its superior performance as a bearing for high speed rotation, suffers from a problem of high costs, compared to a general bearing system utilizing a rolling bearing.
The present invention has been conceived to overcome the above problems and aims to provide a simple-structured controller of a magnetic bearing, for realizing cost reduction and a function equivalent to that which would be obtained through eight-phase controlling.
In order to achieve the above object, according to a first aspect of the present invention, there is provided a magnetic bearing controller. The controller is attached to a magnetic bearing which comprises a stator made of magnetic material having a plurality of pole teeth arranged with an interval space, along an external circumference of a rotor made of magnetic material, three windings connected in star connection or delta connection for exciting the plurality of pole teeth, and an axis displacement sensor for detecting a position displacement of the rotor in two orthogonal directions, and controls current to be supplied to the three windings. The controller comprises a three-phase current value conversion means for computing, in response to receipt of a signal indicative of an axial position displacement in two orthogonal directions, three current values for the three windings, to cause the three windings to generate a magnetic force to serve as a thrust for compensation of the axial displacement position of the rotor.
Also, preferably, the three-phase current value conversion means computes the three current values Iu, Iv, Iw based on the expressions
Iu=F*COS(xcex8)
IV=F*COS(xcex8+2xcfx80/3)
Iw=F*COS(xcex8+4xcfx80/3)
wherein F is a value proportional to a squared value of a force magnitude, A is a directional angle of the force, n is a desirably selected integer, and xcex8 is A/2+nxcfx80.
Further, preferably, the three-phase current conversion means determines a value for the n such that division, by xcfx80, of an absolute value of a difference between xcex8, computed based on a last input axial position displacement, and xcex8, computed based on currently input axial position displacement, leaves a smallest surplus.